it is "open year round" until they have an emergency closure, which they usually do this time of year if the returns of wild fish are low.
wild steelhead in the snoqualmie and most all the other rivers in this state is catch and release only, don't take them out of the water law, etc.
flyfishing has nothing to do with it being open year round.
above the falls the south fork is open year round i believe, it is tough fishing in the winter time for small trout.